This application claims priority to GB Application No. 0606200.4 filed 28 Mar. 2006, the entire contents of which are incorporated herein by reference.
The present invention relates to refuelling/defuelling equipment on board an aircraft, particularly but not necessarily exclusively, an aircraft having onboard refuelling/defuelling equipment comprising internal fuel-flow restrictors.
Aircraft are refuelled at airports between flights with the amount of fuel required to complete their next flight. In order to maximise the revenue generated by an aircraft it is desirable to maximise the flying time of the aircraft and minimise the time spent on the ground between flights. The time spent refuelling an aircraft is a significant contributor to the time required between flights and therefore it is desirable to minimise that refuelling time.
Aircraft are refuelled using ground equipment such as that shown in FIG. 1. An aircraft's refuelling system is connected to a fuel hose 1 by a hose end coupling 2 and fuel is pumped from a fuel storage tank 3 by fuel pump 4 into the aircraft. Two stages of pressure regulation are provided in the ground equipment, namely a first fuel pressure regulator 5 which regulates the pressure to approximately 60 psig and a second adjustable fuel pressure regulator 6, typically integral with the hose end coupling 2, which further reduces the pressure to an adjustable value, typically in the range of 30 to 50 psig.
FIG. 2 shows a conventional refuelling connector 20 for mounting on an aircraft. A fuel outlet 21 connects to the refuelling pipework of the aircraft and fuel inlets 22 and 23 each provide a detachable connection to a hose end coupling 2. The fuel hose used to connect the ground equipment to the aircraft may limit the flow rate of fuel and so two inlets are provided to allow two hoses to be used in parallel, thereby increasing that flow rate. Each refuelling connector has a sealing mechanism 24 such that the inlet is sealed when no hose is connected. The two inlets 22 and 23 combine at isolation valve 25 from where fuel can pass out of the outlet 21 and into the refuelling system of the aircraft. Such a refuelling connector, having two fuel inlets, is typically utilised on larger aircraft where the volume of fuel to be transferred is large. For smaller aircraft a refuelling connector operating on the same principles, but having only one fuel inlet, is often used.
During refuelling, air in the aircraft's fuel tanks is forced out of the tanks through a vent system, venting through flame arrestors to the exterior of the aircraft. Due to resistance to the flow of air in the vent pipework the pressure in the fuel tanks rises during refuelling. If an overflow occurs and fuel is forced through the vent system the pressure increase is higher due to the increased drag of fuel through the vent system. Aircraft fuel tanks are typically designed to have a strength that is sufficient to safely accommodate these pressure increases in the overflow condition. Thus the fuel tanks must be constructed more strongly than would be the case if they simply had to contain fuel. This increase in strength leads to an increase in weight and size of the tanks, which is undesirable as it reduces aircraft performance and design freedom. It is therefore advantageous to limit the pressure increases and thereby limit the added strength and weight required. An aircraft's fuel tanks are thus designed to contain a certain pressure, and that pressure must not be exceeded at any point. During aircraft operation, the highest pressure rise in the fuel tanks may be that sustained during the above-described overflow condition. As such, overflow during refuelling may be the limiting design case.
The pressure rise in the fuel tanks during refuelling is dependent upon the rate of fuel flow into the tanks from the refuelling apparatus. In order to limit the pressure rise during refuelling, fuel flow restrictors are typically included into the refuelling pipework. The fuel flow restrictors are designed to provide, in the overflow condition, the maximum tolerable fuel flow rate at the maximum possible fuel supply pressure. The fuel supply pressure to the aircraft is defined by the adjustable fuel pressure regulator 6 in the hose end coupling of the ground equipment. The typical adjustable pressure regulator 6 is able to be set to limit the fuel pressure to a pressure in the range of 30 to 50 psig. It is therefore assumed that the fuel pressure may be as high as 50 psig. A further safety margin of 10% is then applied to this pressure so that the maximum fuel supply pressure, for the purposes of designing the fuel flow restrictors, is assumed to be 55 psig. The fuel flow restrictors are therefore designed such that the pressure rise in the fuel tanks in the overflow condition is within the required limits at a fuel supply pressure of 55 psig.
Although the maximum supply pressure is assumed to be as high as 55 psig, it is more commonly between 30 psig and 40 psig. At those lower fuel supply pressures the fuel flow restrictors act to limit the flow rate. The refuelling time of the aircraft is therefore longer than could be achieved if the fuel flow restrictors were removed or were less restrictive.